The invention is in the field of transistor amplifier circuits, and relates more particularly to high-frequency amplifier circuits and bias circuits for such amplifier circuits.
In high-frequency amplifier circuits, the output stage is typically a bipolar transistor connected in a common-emitter configuration, biased with either a voltage source or a current source providing a bias signal at the base of the output stage transistor.
In IC technology, a straightforward way of biasing the output stage is to use a current-mirror circuit where the output stage is part of the current-mirror circuit. The quiescent DC current in the output stage can be directly controlled by a constant current in the current-mirror circuit. Typically, the bias of the amplifying transistor is provided by the current-mirror circuit through a resistor. However, the use of a resistor results in an increase in power loss in the circuit. This loss may be quite significant when a large current flows through the resistor.
In order to reduce or eliminate this loss and provide RF isolation between the amplifying transistor and the biasing circuit, it has been suggested to use an inductor instead of a resistor to couple the bias circuit to the base of the RF amplifier transistor, as shown for example in U.S. Pat. No. 6,043,714. However, this is not a complete solution to the problem, since parasitic resistance from the inductor can still cause a power loss and furthermore, it may cause a mismatch between the transistor pair in the current-mirror circuit due to the parasitic resistance. In some IC amplifier designs, these problems can be reduced by employing an external inductor, having a lower parasitic resistance than that of an on-chip inductor, but with many circuits this will require two bonding pads for external connection. This is relatively undesirable, since bonding pads are always at a premium in IC design.
Accordingly, it would be desirable to have a high-frequency amplifier circuit in which a bias circuit is employed which minimizes mismatch, minimizes power loss, and permits an external component connection, if required, to be done with only a single bonding pad.
It is therefore an object of the invention to provide a high-frequency amplifier circuit in which a bias circuit is provided which minimizes mismatch, minimizes power loss due to parasitic resistance in an inductor, and permits an external component to be connected with only a single bonding pad.
In accordance with the invention, these objects are achieved by a new high-frequency amplifier circuit which includes an amplifying transistor and a bias circuit directly connected to the amplifying transistor. The bias circuit includes a bias transistor having a control terminal and an inductor coupled to the control terminal, with the bias transistor also having an output terminal directly connected to the amplifying transistor.
In a preferred embodiment of the invention, a resistor is connected in series with the inductor coupled to the control terminal of the bias transistor, and the resistor and inductor are coupled between the control terminal and a power supply terminal.
In a further preferred embodiment of the invention, an additional transistor is provided having a control terminal connected to the output terminal of the bias transistor and an output terminal connected to the control terminal of the bias transistor.
High-frequency amplifier circuits in accordance with the present invention offer a significant improvement in that mismatch effects and power losses due to parasitic resistance are minimized and the connection of an external component such as an inductor is simplified.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments to be described hereinafter.